Rasmussen delivered a speech to his audience with three points: defense matters, defense spending, and the enduring bond between Europe and North America.

When talking about the situation in Ukraine, Rasmussen said "defense matters for our Allies, and NATO is needed as much as ever."

"We have taken immediate measures to strengthen our collective defense. We already have more planes in the air, more ships at sea, and more exercises on the ground," he added.

The NATO secretary general expressed his concern on defense spending of the members countries.

He stressed European allies has cut the spending by almost 10 percent in real terms since 2008.

"To keep our defences strong, we need to start reversing this trend. We need to spend more on defence, and we need to spend better on defence," he continued.

When illustrating his views on transatlantic bond, Rasmussen said it "remains the bedrock of our freedom, our prosperity and our way of life."

"It is critical that we all continue to invest in it politically, militarily and financially," he said during the meeting in Vilnius.

Though Europe and North America enjoy deep historical, political and economic ties, according to Rasmussen, he still reminds his audience that "we cannot take each other for granted."

"I look to you, as parliamentarians, to continue playing a key role in fostering a common transatlantic spirit," he said.

Some 600 representatives from 28 NATO members, NATO PA associate countries, regional partners, delegations of the European Parliament and other international organizations are participating in the three-day session in Lithuania, according to the country's parliament, the Seimas.

Founded in 1955, the NATO PA serves as the consultative interparliamentary organization for NATO. The spring session of NATO PA this year comes after elections of the European Parliament and Lithuania's president on Sunday.